The only thing about if it were true of getting 400's is then they would have a difference in pax numbers,say one a/c was to go tech then quite a bit of re-arranging has to be done to squeeze a 400's pax onto a 300,if you understand me.Much easier for a loco just to have total fleet commonality.
As for turnarounds if airlines like BMI for example at Belfast City were to close their check-inn like Easy's 30mins beforer departure they can be turn round in just as tight a time.We got a BMI A321 turned round back for Heathrow in 23 minutes as it was timed.
Regards